1. Field of the Invention
The present invention relates to a knocking determining apparatus of an internal combustion engine, and more specifically, to a knocking determining apparatus of an internal combustion engine including an in-cylinder injector injecting a fuel into a cylinder and an intake port injector injecting a fuel into an intake manifold or an intake port.
2. Description of the Background Art
In general, what is called a dual-injection-type internal combustion engine including an in-cylinder injector injecting a fuel into a cylinder and an intake port injector injecting a fuel into an intake manifold or an intake port is known (Japanese Patent Laying-Open No. 2001-020837 and the like), in which use of these injectors is switched in accordance with an operation state of the engine so as to realize stratified charge combustion in a low-load operation region and homogeneous combustion in a high-load operation region and so as to inject the fuel at a prescribed injection ratio in accordance with the operation state, for achieving improvement in fuel efficiency characteristic and output characteristic.
Normally, in an combustion engine, knocking determination for determining whether or not knocking has occurred is performed, and based on the result, what is called knocking control is executed, in which ignition timing and the like is controlled. The knocking determination is performed using a knock sensor that is a vibration detecting sensor arranged at a cylinder block or the like and based on an output signal of the knock sensor after ignition in each cylinder, whereby occurrence of knocking is determined (Japanese Patent Laying-Open No. 07-103048 and the like).
Generally, an injector injecting fuel in an internal combustion engine is configured to open a valve to start fuel injection by providing an electric current to an electromagnetic solenoid so that a nozzle needle is driven to be separated from a valve seat, and to close the valve to stop fuel injection by stopping electricity to the electromagnetic solenoid so that the nozzle needle is seated on the valve seat. Such an injector, with its open-close operation, generates vibrations, e.g., hitting noises when the nozzle needle is seated on the valve seat. The vibrations generated in accordance with the open-close operation of the injector may be superimposed on an output signal of the knock sensor as operation noises. In particular, as the in-cylinder injector is arranged closer to the knock sensor as compared to the intake port injector, the operation noises generated by the open-close operation of the injector tend to impose greater effect on the output signal of the knock sensor. Further, when the combustion speed of an air-fuel mixture is accelerated due to a change in a fuel injection ratio of the in-cylinder injector to be increased, the vibrations occurring in accordance with combustion of the air-fuel mixture are increased. This also tends to contribute to increase the engine vibration level.
Accordingly, in the aforementioned internal combustion engine, for example, as described above, since the engine vibration level and the like also changes when fuel injection from the intake port injector is switched to fuel injection from the in-cylinder injector, or when an injection ratio of the in-cylinder injector is changed to increase, an output value of the knock sensor abruptly increases. Although the level of background noise (the normal vibration level) gradually rises also, it may be erroneously determined that knocking has occurred, while the knocking has not occurred actually. As a result, ignition timing may be erroneously retarded, which may impair drivability.
Conversely, when fuel injection from the in-cylinder port injector is switched to fuel injection from the intake port injector, or when an injection ratio of the intake port injector is changed to increase, an output value of the knock sensor abruptly decreases. Although the level of background noise gradually falls also, it may be erroneously determined that no knocking has occurred, while the knocking has occurred actually. As a result, ignition timing may be erroneously advanced, which may cause further greater knocking.